1. Field of the Invention
The present invention relates to a control apparatus for an electric motor which uses a reluctance torque primarily and utilizes a magnet torque secondarily (in an auxiliary manner) and which is applied to a machine tool or the like, and more particularly to a control apparatus for a reluctance type synchronous motor in which, in order to compensate for nonlinear characteristics of the actual torque of the motor arising relative to a torque command value, a q-axis current command value is compensated for by a compensation value calculated on the basis of a d-axis current command value and a rotor speed.
2. Related Art
There have hitherto been known a control apparatus and a control method for a reluctance type synchronous motor (hereinbelow, simply termed the “motor”), wherein the magnitude of a field current such as d-axis current is changed in proportion to a torque command value as in JP-A-11-356078. FIG. 10 shows a control block diagram of such a motor applied to the feed shaft control or spindle of a general machine tool, and it illustrates a configuration similar to that of a torque-current conversion section in JP-A-11-356078. Especially, a portion which calculates a q-axis current command value and a d-axis current command value from a torque command value in a general d-q axis vector control is omitted for clarity of description. Next, the flow of processing will be explained. The torque command value STC is input from a host controller, not shown, to a q-axis current calculation unit 1, whereby a q-axis current amplitude value SIQC is calculated. A q-axis current coefficient SKIQ is calculated by a q-axis current coefficient calculation unit 4 in accordance with a rotor speed SPD, and the q-axis current amplitude value SIQC and the q-axis current coefficient SKIQ are multiplied by a multiplication unit 5, to thereby generate the q-axis current command value SIQCC. Here, the q-axis current command value SIQCC agrees in sign with the torque command value. The torque command value STC is then input to a d-axis current calculation unit 2, whereby a d-axis current amplitude value SIDC is calculated. A d-axis current coefficient SKID is calculated by a d-axis current coefficient calculation unit 3 in accordance with the rotor speed SPD, and the d-axis current amplitude value SIDC and the d-axis current coefficient SKID are multiplied by a multiplication unit 6, to thereby generate the d-axis current command value SIDCC.
FIGS. 7(a), 7(b) and 7(c) show functional examples of the d-axis current coefficient, the q-axis current coefficient, and a q-axis compensation current coefficient, respectively, as a applied to a control apparatus for a motor. As shown in FIGS. 7(a) and 7(b), the q-axis current coefficient SKIQ and the d-axis current coefficient SKID normally become “1” when the rotor speed SPD is below a basic rotation speeds the SPDB, and become larger or smaller than “1” when the rotor speed SPD is equal to or greater than the basic rotation number SPDB. However, they sometimes become “1” at or above the basic rotation number SPDB, depending upon the characteristics of the motor.